This invention relates to "bed-of-nails" test fixtures for the electrical testing of printed circuit boards. In this testing, the circuit board is brought into electrical contact with a number of test probes, placed so as to contact predetermined test nodes on the board. Testing of the entire circuit, or of the individual circuits and components, or both, can then be carried out by means well known to those skilled in the art.
In circumstances when large numbers of circuit boards have to be tested, such as in assembly line production of widely used circuit boards, it is important that the test procedure can be carried out quickly. It is therefore important that the circuit board can be readily placed in and removed from the test fixture, with a minimum of operator skill required.
However, it is also very important that accurate registration is obtained between the probes and the corresponding test nodes on the circuit board. This registration must be repeatable over a large number of test cycles. Accuracy of registration is becoming increasingly more important as modern techniques allow greater component densities on the boards.
Other features advantageous in such test fixtures are ease of access to the probes and other test equipment, and reliability of operation.
In general, when the board is first placed in the test fixture, it is not in contact with the probes. Once in position, it is brought into contact with the probes so that the testing can begin. The simplest type of test fixture provides this contact by mechanically pressing the board onto the probes, using an assembly which provides evenly distributed pressure to the board, and which is mounted over the board. A major disadvantage of this system is that this assembly must be put in position after the board has been placed in the test fixture, and removed before the board can be removed from the fixture. These extra steps significantly increase the time required for loading and unloading the board, and increase operator fatigue. Another disadvantage is that the assembly on top of the board means that it is not possible for the operator to have access to components on the board during the test process. Such access is desirable in certain test operations.
Most commercially available test fixtures use a partial vacuum induced in the space between the board and the test bed in order to produce the relative movement of the two in order to bring the board into contact with the probes. In order to achieve this, a flexible airtight seal must be provided around the board. This has generally been provided by a rubber seal, with a spring means of returning the board to the upper position when the vacuum is released.
It is vitally important for successful testing that the board accurately registers with the probes when they are brought into contact. This is achieved by designing the fixture so that the board is accurately positioned relative to the probes when it is placed on the fixture, and by providing means which accurately guide the board and probes into contact, maintaining this accurate positioning. It can readily be appreciated that to do this the possibility of lateral movement of the board must be minimized. Further, the plane of the board, when it reaches the contact position, must be parallel to its original plane.
A number of ways of achieving this registration have been used in the prior art. In one, the board to be tested is the part which moves, and the guidance is provided by at least two tooling pins, which are mounted in and are perpendicular to the platen which carries the test probes. The printed circuit board to be tested has tooling holes, which are also used in the process of fabricating the board. The tooling pins are arranged so that the board tooling holes fit over them when the board is correctly positioned over the test probes. The movement of the board into contact with the probes is then guided by the tooling pins.
In the case of boards which are not provided with tooling holes, a similar system can be used using edgeguidance pins in place of tooling pins.
This use of tooling guide pins to produce guidance has a number of disadvantages. In use, the board can stick on the pins, but if too much play is allowed for the holes passing over the pins, lateral movement becomes possible. It is also possible for the board to twist during movement, so that it is no longer parallel its original plane, resulting in misalignment. Further, through use the pins can become misaligned or worn, decreasing the accuracy of registration. There is also an inherent fragility in the system, as the size of the tooling holes, and hence the thickness of the tooling pins, is limited by constraints of board size and design. The pins can be damaged if the operator is careless in placing the board, or if the board is forced onto the pins. Even slight displacement of the tooling holes can cause similar problems.
Another prior art means of producing registration is the use of linear bearings to guide the relative movements of the board and the platen. These bearings have similar problems of wear, fragility, sticking if not properly lubricated, and the possibility of the board becoming nonparallel during movement. As these bearings must have an internal clearance in order to operate, there is also the possibility of lateral movement. They must be handled carefully during disassembly of the fixture for maintenance purposes because any damage seriously impairs their effectiveness. The accurate placing of the board on the test fixture is ensured in this system by the use of short tooling pins to fit through the board tooling holes.
A third prior art system uses a fixed board, and moves the platen carrying the probes to bring the test probes into contact with the board. In this system the registration is achieved by guiding the probes through holes in an upper, non-moving platen. The registration in this system is inherently inaccurate because of the tolerance required in the upper platen holes in order to permit the probes to move through freely. This system has other disadvantages, in particular the holes often become clogged with debris from the board, and extremely accurate alignment between the pin positions on the bottom platen and the corresponding holes on the upper platen must be achieved, or a serious loss of registration accuracy results.